by Doyle Rice, USA TODAY

by Doyle Rice, USA TODAY

For the first time in recorded human history, levels of carbon dioxide in the atmosphere have surpassed 400 parts per million (ppm), according to data released Friday morning from the National Oceanic and Atmospheric Administration (NOAA) from the Mauna Loa Observatory in Hawaii.

The average level of carbon dioxide over the past five days is 400.03 ppm. Carbon dioxide (CO2) is the greenhouse gas that is responsible for 63% of the warming attributable to all greenhouse gases, according to NOAA's Earth System Research Laboratory in Boulder, Colo.

Increasing amounts of carbon dioxide and other gases caused by the burning of the oil, gas and coal that power our world are enhancing the natural "greenhouse effect," causing the planet to warm to levels that climate scientists say can't be linked to natural forces.

Carbon dioxide levels were around 280 ppm prior to the Industrial Revolution, when we first began releasing large amounts into the atmosphere through the burning of fossil fuels.

Carbon dioxide levels were closer to 200 during the Ice Age. There are natural ups and downs of this greenhouse gas, which comes from volcanoes and decomposing plants and animals. But that's not what has driven current levels so high, said NOAA senior scientist Pieter Tans of the Earth System Research Lab. He said the amount should be even higher, but the world's oceans are absorbing quite a bit, keeping it out of the air

"That increase is not a surprise to scientists," Tans said. "The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration."

During the last 800,000 years, CO2 fluctuated between about 180 ppm during ice ages and 280 ppm during interglacial warm periods. Today's rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended.

The last time that carbon dioxide reached 400 ppm was millions of years ago. How do scientists know this?

Scientists can analyze the gases trapped in ice to reconstruct what climate was like in prehistory, but that record only goes back 800,000 years, according to the Scripps Institute of Oceanography in San Diego. It is harder to estimate carbon dioxide levels before then, but in 2009, one research team reported in the journal Nature Geoscience that it had found evidence of CO2 levels that ranged from 365 to 415 ppm, roughly 4.5 million years ago.

"They based their finding on the analysis of carbon isotopes present in compounds made by tiny marine phytoplankton preserved in ancient ocean sediments," according to Scripps.

"Crossing the 400 ppm threshold is more than a new data point about greenhouse gas levels in our atmosphere," World Wildlife Fund chief scientist Jon Hoekstra says. "It's a sobering reminder that the planet we know today will not be the planet we know tomorrow."